It seems to me the NM_DEVICE_CLASS_DECLARE_TYPES() macro confuses more
than helping. Let's explicitly initialize the two fields, albeit with
another helper macro NM_DEVICE_DEFINE_LINK_TYPES() to get the list of
link-types right.

For consistency, also leave nop-lines like

  device_class->connection_type_supported = NULL;
  device_class->link_types = NM_DEVICE_DEFINE_LINK_TYPES ();

because all NMDevice class init methods should have this same
boiler plate code and to make it explicit that this is intended.
And there are only 3 occurences where this actually comes into play.